Resin cellular molded articles having a skin are used, for example, as materials for automobile interiors or the like. More specifically, examples of automobile interior trim material uses are door material, instrument panel, sun visor, ceiling material, door trim, rear package, knee bolster, air bag door, head rest, arm rest, various pillars, quarter trim, front side trim, front seat back, crash pad, console box, console head, luggage floor cover, partition board, center console, console box cover, and so forth.
Previously, articles such as are formed by pressure molding, in a heat-softened state in a mold, of laminates made by sequentially superimposing an imitation leather sheet such as polyvinyl chloride, a foam sheet such as polypropylene foam and a core material such as ABS resin and binding these together into one unit with adhesive, are known as automobile interior trim materials (Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 54-158471). However, said automobile interior trim material having a foam sheet feels hard to the touch when pressed, and further disadvantages are that the cushioning property is inferior and there is no feeling of depth.
As a means to solve the disadvantages of said automobile interior trim material, a united laminate made by the sequential stacking of an artificial leather sheet made of polyvinyl chloride, a foam sheet such as polypropylene foam, a layer of expanded polyurethane and a core material such as ABS resin is known. As a result of having the intermediate layer of expanded polyurethane, said laminate has both an undoubtedly soft touch given to it when pressed, and an improved cushioning property compared to said automobile interior trim material. However, it is heavier compared to this automobile interior trim material, and moreover, at high temperature a foul odor is emitted due to the polyvinyl chloride, and a further disadvantage is that a harmful gas is produced during burning.
Furthermore, in recent years in the automobile industry, in order to improve the automobile fuel consumption, there has been a great demand for the lightening of automobile constructing parts, and in line with this, still further lightening is being sought. Furthermore, from the viewpoint of the reclamation of resources, the development of an easily recyclable automobile interior trim material is long-awaited. Accordingly, in consideration of said prior art, the present inventors carried out accumulated extensive studies in order to develop an automobile interior trim material which is able to simultaneously satisfy the following numerous demands: (1) a soft and supple feel is given when pressed, (2) the cushioning property is superior, (3) there is a feeling of depth, (4) a foul odor is not produced at high temperature, (5) the amount of noxious gas produced during burning is small, (6) lightening can be promoted, and (7) recycling is easy.
Conversely, a cellular molded article of a polypropylene resin on which is provided a skin material, such as an automobile interior trim material, is produced by a method in which a cellular molded article is fabricated in advance, adhesive, for example, is applied to this and then the skin material is stuck on. However, a disadvantage is that when using this method, since three steps are needed consisting of the fabrication of a molded foam, the application of adhesive and the sticking of the skin material to the molded foam, the manufacturing process is troublesome and the production efficiency is poor.
As a method for solving the disadvantages in the above-mentioned process, a method for manufacturing a unified cellular molded article is known (Japanese Unexamined Publication (Tokkyo Kokai) No. 3-190723, and so forth), in which a porous sheet having gas permeability is set up on at least the face having vapor holes of a pair of dies having a plurality of vapor holes, then after charging with expanded particles of a thermoplastic resin, steam is introduced through the vapor holes, and the porous sheet and the expanded particles are melt bonded by heating.
A problem with the aforesaid production method of a cellular molded article is that, while the method is effective when the skin material is a porous sheet, if the skin material is a resin sheet or the like which has no gas permeability, since the steam introduced from the vapor holes is impeded by this resin sheet and it does not reach the expanded particles, the expanded particles are not sufficiently heated. This gives rise to poor melt bonding, and the mechanical action of the obtained molded article is inferior to the case where the skin material is bonded to the molded foam using adhesive.
Accordingly, the present inventors, in consideration of said prior art, carried out extensive studies in order to obtain a polypropylene resin cellular molded article having a skin, wherein even though the skin has no gas permeability, it is possible to unite the skin material with a molded foam simultaneously with the production of the molded foam from pre-expanded particles, and moreover, wherein the melt bonding in the molded foam is not inferior. As a result, without thorough heating of one side of the dies, using pre-expanded particles of a polypropylene resin, and moreover, when performing thermal expansion of the pre-expanded particles under special conditions, even in the case where the thickness of the foam layer is large, it was discovered how to obtain a cellular molded article having a skin in which there is no occurrence of inferior melt bonding or the like, and the present invention was realized.